Showing papers by "Sankaran Sundaresan published in 1987"

The role of lattice oxygen in the dynamic behavior of oxide catalysts

Abstract: The lattice of an oxide catalyst used for oxidation reactions can act as a reservoir for oxygen, storing and releasing it for reactions at the catalyst surface under appropriate conditions. The implication of this oxygen storage property of an oxide catalyst on its dynamic response characteristics has been investigated through an experimental study of 2-butene oxidation over vanadium oxide as a model reaction. Isothermal reaction rate measurements in a differential reactor and nonisothermal studies in a single pellet reactor have been carried out. Following a step increase in the feed butene concentration, isothermal reaction rate overshoot and pellet temperature overshoot were observed. These observations could be modelled in a qualitatively correct way by a very simple model accounting for the participation of lattice oxygen in the catalytic reactions under dynamic conditions. It is demonstrated through model simulations that the ignition characteristics of a catalyst pellet are significantly affected b...

Optimal catalyst distribution and dilution in nonisothermal packed bed reactors

Abstract: The problem of the optimal distribution of two different catalysts in a nonisothermal packed bed reactor is analyzed. State variable constraints, such as maximum allowable temperature, may be incorporated through a penalty function approach into the steepest ascent solution of this problem. Loading a wall-cooled reactor with two chemically different catalysts is compared to dilution of a single catalyst with inerts for temperature moderation. A criterion is presented for estimating whether catalyst dilution may be necessary in order to keep reactor temperature below some maximum allowable value. Conditions for optimal loading with a two-dimensional reactor model are presented and applied to catalyst dilution in a butane oxidation reactor. Suboptimal catalyst loadings are compared with the optimal loading.

Mathematical modeling of pulsing flow in large trickle beds

Abstract: A crossflow model for pulsing flow in a trickle bed of large cross section is presented. The trickle bed is idealized as being made up of pulses and the trickling flow region surrounding these pulses. Assuming the macroscopic gas flow around the pulses to be irrotational and the pulses to contain only liquid, the model equations are derived. An analysis of the model and comparison with the experimental results of Christensen (1986) reveals that the irrotational flow model underestimates the pulse velocity for realistic pulse shapes. This can be attributed to the underestimation by the model of the drag exerted by the trickling region on the pulses. By dropping the irrotational flow assumption and viewing the model as semiempirical, a single parameter characterizing the magnitude of this drag is estimated from the pulse velocity data. When this estimated value for this parameter is employed, the model predicts the average pressure gradient data reasonably well.

Real-time study of self-sustained oscillations in the CO oxidation rate on Pt

Abstract: The driving mechanism for self‐sustained oscillations in the rate of some catalytic oxidations under steady‐state conditions has remained unexplained despite numerous theoretical and experimental studies since the early 1970s. We have used reflection–absorption infrared spectroscopy to perform the first real‐time analysis of a Pt foil surface during constant temperature oscillations in CO oxidation. The CO absorption capacity of the surface is shown to vary periodically during oscillations. Inverse correlation of the CO adsorption capacity with surface carbon concentration (measured with Auger spectroscopy), and demonstration that (1) carbon can diffuse from the bulk to the surface, (2) oxygen can remove surface carbon from the surface, and (3) CO can block carbon diffusion, all at oscillation temperatures, indicate that variation in surface carbon concentration may be driving the oscillations. Modeling of the phenomenon is found to account for all observations.

Limitations of the equilibrium theory of countercurrent devices

Abstract: In the analysis of countercurrent adsorption processes, an equilibrium theory in which local equilibrium between the fluid and the adsorbent phases is assumed has been used in the past to determine the composition of the pinch state that would be obtained for long columns. It is shown that this equilibrium theory is deficient and that certain mass transfer parameters, which are lost in the procedure by which the model equations for the equilibrium theory are formulated, can have a profound effect on the composition of the pinch.

Self-sustained Kinetic Oscillations in the Catalytic CO Oxidation on Platinum

Abstract: The observation of self-sustained oscillations in the rates of catalytic reactions under specific catalyst temperature is widespread [1]. It includes the oxidations of hydrocarbons, H2, CO and other gases over Pt, Pd, Ir and Ni surfaces. In general, oscillations are observed on supported catalysts or foils in relatively high pressures (1–1000 Torr) and are characterized by long periods, ranging from several minutes to several hours. However, relatively rapid oscillations (≈ seconds) have also been observed on clean Pt(100) crystals at low pressures (≈10−4 Torr), and attributed to adsorbate-induced transformation of the surface structure [2]. Variations in surface reconstruction clearly cannot be involved for the majority of the systems under study since most do not reconstruct and all contain surface impurities under oscillation conditions. As a result, the driving mechanism for the majority of systems still remains poorly understood.